IT Placement Practice Test 6

Let n1 liters of 90% concentration has to be mixed with n2 liters of 75% concentration to get 78% concentration solution.
Using weighted average formula ax = ( n1 × a1 + n2 × a2 )/ (n1 + n2)
⇒ 78 = (n1 × 90 + n2 × 75) / (n1 + n2)
⇒

n1

n2

= 1/4.
By dividing 30 in the ratio 1:4 we get 6 : 24. So we need 6 liters.